Fix get_cursor() for max-sized block

Fix get_cursor() for max-sized block

Since the size of a max-sized block overflows hed_uoff_t, its tree
node appears to be a zero-sized block and is not returned by
find_in_tree().

First, this is the only way find_block() can ever fail.

Second, there can be only one max-sized block in a file: either if
the file is empty, or if its size is MAX_UOFF + 1. Note that the
latter case is not yet handled, and not even made possible by the
operating system for regular files.

Consequently, if block search fails, it must be because there is a
max-sized block somewhere in the chain, possibly preceded by one or
more zero-sized insert block(s) and/or followed by a zero-sized
terminal block.

In other words, the max-sized block must be either:

  a. the terminal block (for empty files), or
  b. the block just before the terminal block.

Signed-off-by: Petr Tesarik <petr@tesarici.cz>